Though we meet at lunchtime in a cafe, Matthew Macfadyen chooses not to order any food. His explanation is that he had breakfast late, but it may be that he is getting into shape for one of the more unlikely nude scenes in cinema history: his unclothed role as John Birt, the former BBC director-general and special adviser to Tony Blair.

The film is Frost/Nixon, Ron Howard's version of the Peter Morgan play that starts shooting in California at the end of the summer. In the London and Broadway stage version, one speech refers to the fact that Birt, producer of David Frost's interviews with the disgraced president, was so thrilled by the success of the interview dealing with Watergate that he took off his clothes and dashed into the Pacific. But cinema must show where theatre can merely tell - and so Macfadyen will portray Birt in the buff.

The women sneaking glances from neighbouring tables indicate that at least some viewers would accept the actor in his birthday suit right now, but movies, Macfadyen suggests, are a cruel mirror. Men don't quite suffer from the size-zero rules imposed on female actors, but firm flesh is still enforced, as he discovered in his most high-profile film role to date, opposite Keira Knightley in the big-screen version of Pride and Prejudice.

"The studio gave me a personal trainer," he says. "Which is sort of fair enough because you don't want a flabby Darcy. But it was quite a shock. As soon as I wasn't in a scene, I'd be taken running round the park. And I was put on a low-fat diet. Every morning, this black cool-box would arrive with everything I was allowed to eat for the day. It was reassuring to find that I could get in shape quickly if I needed to, but it made you think about what women go through in this business."

In the theatre, producers allow more leeway on appearance, though the character Macfadyen, 32, is currently playing is the kind of guy who would certainly employ a personal trainer. Clay, in the London premiere of Bruce Norris's New York hit The Pain and the Itch, is a Brooklyn Heights liberal whose interpretation of fatherhood involves being a near-presidential-level bodyguard to his young daughter. He believes his money and love can protect her from anything. But the child is suffering an intimate dermatological condition, referred to in the title, and this leads Clay to face the terrifying possibility that his bien-pensant household has been invaded by a rodent, a racist and a paedophile.

In rehearsal, the main problem has been learning and delivering Norris's often minimal dialogue. A line might run, "Uhhh ... yeah ... uhhh ... no ... " in an American register. "I haven't done an accent since drama school, and it is very difficult," Macfadyen says. "It literally comes from a different place in your mouth. It's odd because you grow up listening to American accents all your life."

He points out, though, that his Brooklyn Heights impression is relatively easy compared to the vocal feat required of Amanda Boxer, who plays Clay's mother. At one point, she has a speech about how much better British film actors are than American ones, complete with supporting quotations. So, during those lines, Boxer is a Brit pretending to be an American pretending to be a Brit.

Until The Pain and the Itch, all Macfadyen's performances have utilised the slightly soft, somewhat posh tones that he developed during travels in the far east as the son of an expat oilman, before being sent as a boarder to Oakham School in Leicestershire. The acting DNA seems to come from a mother who taught drama and a grandfather who produced amateur plays. This background has helped him become the voice of the handsome English establishment: before Pride and Prejudice, there were three series as Tom, the charming guardian of national security, in BBC1's Spooks.

During Spooks rehearsals, he and the other actors - including Peter Firth, who played his MI5 boss - were taken to meet real agents. "We met an ex-MI5 and an ex-CIA man called Mike and Nick. Oh yeah, I bet that's what they were really called. Peter got redder and redder and finally asked this incredibly laidback guy, 'Have you ever used the, er, final option on someone?' And Nick said, 'It's a grey area.' And we all looked at each other."

Macfadyen found this seminar in spycraft amusing, though is not sure how useful it was. For the recent Channel 4 film Secret Life, in which he delivered a powerful performance as a paedophile, he declined an offer to meet actual offenders at a rehab centre. Many actors of his generation are drawn to the idea, imported from the US, of "immersion research". It's a method that falters, Macfadyen believes, when one is cast, say, as a paedophile or a rapist. "More and more I think the key to acting is keeping it simple: it's about recreating human behaviour through a series of actions. But actors tend to get scared and think they aren't doing enough, and so it all becomes too big. I'm not getting at other people. I've felt myself doing it."

For Pride and Prejudice, one piece of research Macfadyen decided not to do was watch DVDs of Colin Firth's television version of Mr Darcy, in which the actor famously bathed in a lake semi-clothed: "I had seen his wet-shirt scene at an awards ceremony, but I hadn't really registered the impact of it. So I was unprepared for the 'How are you going to make it different?' question. I think I got rather precious with a journalist who asked that. But I just thought, 'You're missing the point.' It was no different from playing, say, Hamlet after all these other people had done it."

Polite and bright, Macfadyen seems more shy than you expect a leading actor to be. Even mildly personal questions are greeted with a blush that makes the rest of his face match the tip of his nose, which has been burned by early-summer sun. This reticence seems partly rooted in his memories of being a young theatre-goer: "I remember when I was little, theatre actors were much more mysterious and fascinating. You'd see them come out of the stage door and think, 'Where are they going?' Now, we know everything. I think it's important not to know too much about actors. I don't care what Sean Penn thinks about Iraq. Because when you see them in the magazines every day, you don't want to go to see them act."

The caution is also derived from his brief experience of being paparazzi fodder. When Spooks was topping the ratings, he began a relationship with co-star Keeley Hawes, and they had some months of being snapped hand-in-hand in London streets. They live with their young children Maggie and Ralph, and Myles, Hawes's son from her first marriage. Are there still snappers at the gate? "We're pretty much left alone now. There was one time, I think when we were going in to hospital to have Maggie, and there was a photographer outside the house. He gave us flowers and asked us for a photograph, and we said, 'Is it OK if we don't?' He said yes, and we took the flowers and he went away."

Macfadyen and Hawes recently auditioned separately for Frank Oz's film Death at a Funeral, yet were eventually cast as husband and wife. To a non-actor, this seems one of the more bizarre parts of the job: pretending to be involved with someone you are actually involved with. "Yeah. You think, in advance, it's going to be very strange," he says. "But actually it isn't. You can just separate it off."

·The Pain and the Itch is at the Royal Court, London, from tomorrow until July 21. Box office: 020-7565 5000.